Regarding fuel cell systems mounted on automobiles, etc., various hybrid fuel cell systems each having, as a power source, a fuel cell and a battery, have been proposed in order to deal with a rapid change in a load that exceeds the power generation capability of the fuel cell.
In a hybrid fuel cell system, an output voltage of a fuel cell and an output voltage of a battery are controlled by a DC/DC converter. As a DC/DC converter for carrying out such a control, a DC/DC converter having a configuration in which a switching element such as a power transistor, IGBT or FET is made to perform a PWM operation for voltage conversion has been used widely. Together with reduced power consumption, reduced size and increased performance of electronic equipment, a reduced loss, increased efficiency and reduced noise of the DC/DC converter have further been demanded. In particular, reductions of a switching loss and a switching surge that are associated with the PWM operation have been demanded.
One of the techniques for reducing the above switching loss and switching surge is a soft switching technique. Here, soft switching refers to a switching method for realizing ZVS (Zero Voltage Switching) or ZCS (Zero Current Switching), the soft switching involving a low switching loss of a power semiconductor device and a low stress given to the semiconductor device. Meanwhile, a switching method for directly turning on/off a voltage/current with a switching function of a power semiconductor device is referred to as hard switching. In the descriptions below, a method in which both or one of ZVS and ZCS is realized is referred to as soft switching, whereas the other methods are referred to as hard switching.
Soft switching is realized by a general pressure increase/decrease DC/DC converter provided with, for example, an inductor, a switching element and a diode, the DC/DC converter being additionally provided with an auxiliary circuit for reducing a switching loss (the so-called soft switching converter) (see, for example, Patent Document 1).
Meanwhile, in order to realize enhanced speed, increased capacity and reduced ripple, a multiphase DC/DC converter (multiphase converter) in which a plurality of DC/DC converters are connected in parallel has been used in the related art.
Regarding such a multiphase converter, when employing a soft switching converter as a converter of each phase, this can attain the enhanced speed and increased capacity, but might lead to upsizing of the converter. In light of this problem, it can be considered that a component of an auxiliary circuit included in the soft switching converter of each phase, e.g. an auxiliary coil, can be made to serve as a common coil shared by the converters of the phases. Accordingly, downsizing of the multiphase soft switching converter can be attained.